It is known that a Diesel engine is generally equipped with a direct-injection system that comprises a plurality of electrically controlled fuel injectors, for injecting the fuel directly into the cylinders. The fuel injection in each cylinder is generally performed according to a multi-injection pattern, which comprises a plurality of injection events per engine cycle, including at least one pilot injection and one main injection.
Conventional fuel injection systems are designed for actuating each fuel injector as an on-off valve, so that the fuel injection rate during each injection event is substantially constant, and the fuel quantity injected per event depends principally on the opening period of the injector, usually referred as energizing time. In these systems, the injection strategy provides for controlling the combustion phase within each cylinder by properly adjusting the energizing time of each injection event and/or the dwell time between each couple of consecutive injection events of the same multi-injection pattern, whereby the dwell time is defined as the time period between the beginning of the electric signal responsible of the first injection event and the beginning of the electric signal responsible of the second injection event of the couple.
In order to comply with tighter emission regulations, most car makers are developing an injection strategy oriented towards the control of the combustion phase by replacing one or more injection events of the multi-injection pattern, typically the pilot and the main injections, with a single injection event, and by varying the fuel injection rate during the energizing time of said single injection event in a controlled manner. This strategy is generally referred as injection rate shaping, because the variation of the fuel injection rate causes a change in the shape of the fuel injection rate curve, whereby the fuel injection rate curve is defined as the curve resulting from plotting the fuel injection rate versus the energizing time.
This injection rate shaping achieves the benefit of reducing the combustion noise keeping the same level of polluting emissions and fuel consumption, or conversely of reducing the polluting emissions and fuel consumption keeping the same combustion noise. However, the injection rate shaping is currently applicable only to fuel injection systems equipped with Piezoelectric Direct Acting injectors.
A Piezoelectric Direct Acting injector is designed so that the injector needle is directly fixed to a piezoelectric actuator, which can effectively move the needle in a plurality of different positions, to thereby accurately adjusting the opening degree of the injector and then the fuel injection rate. On the contrary, conventional solenoid fuel injectors are actuated by an electro-mechanical device that can move the needle only in an open position or in a closed position, so that it is practically impossible to voluntarily regulate the opening degree of the injector for varying the fuel injection rate. Nevertheless, it has been found that a kind of fuel rate shaping can be realized also with a solenoid fuel injector, by means of the hydraulic combination or fusion of at least two consecutive injection events of the same multi-injection pattern, typically a pilot and a main injection.
The hydraulic fusion is achieved by reducing the time difference between the two injection events, whereby this time difference can be calculated as the difference between the dwell time separating the first and the second injection event and the energizing time of the first injection event. In greater detail, the hydraulic fusion is achieved by reducing said time difference to a value so small, typically less than approximately 100 μs, that the solenoid injector does not have enough time to completely close before it is commanded to open again. However, the positions assumed by the injector needle in this situation depend on so many factors, including all the mechanical forces acting on the needle itself, the manufacturing tolerances, as well as the fuel temperature, that it is practically impossible to precisely achieve a desired fuel injection rate. Moreover, it has been found that the sensitivity of the total fuel delivery to the timing distance between the two injection events is dramatically high, so that a little drift in injectors' mechanical and/or electric characteristics often results into a strong fuel delivery drift. As a consequence, the resulting injection rate shaping is generally not reliable.
In view of the foregoing, at least one object is to provide a strategy for operating the injection system of an internal combustion engine of a motor vehicle. At least another object is to provide a strategy capable of performing a kind of injection rate shaping, which is sufficiently reliable also in case of solenoid injectors. At least yet another object is to achieve the above mentioned goals with a simple, rational and rather inexpensive solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.